Our studies have shown a good correlation between the known characteristics of NK-CMC and the characteristics of NKCF. These characteristics are: (1)\the demonstration that NKCF is selectively cytotoxic to NK-sensitive target cells; (2)\the demonstration that strains with low NK generate poor NKCF activity; (3)\interferon-treated target cells that are resistant in NK-CMC are unable to stimulate NKCF production; (4)\pretreatment of NK cells with interferon enhances the cytotoxic activity and the production of NKCF; and (5)\a good correlation in both NKCF and NK-CMC reactions of resistance to lysis by variant target cells. Based on these studies, we have developed a model that suggests that the effector cells must be able to recognize and bind to the target antigen as well as stimulate the release of NKCF. The NK-sensitive target cells must be able to bind to the effector cell, stimulate the release of NKCF, bind NKCF, and also be susceptible to lysis by NKCF. We have also been able to generate and characterize NKCF from various sources, namely, mouse, rat, and man. The NKCF has been shown to be sensitive to pH2, trypsin/sensitive, insensitive to 2-mercaptoethanol, but sensitive to alkylation. These results suggest that NKCF is a glycoprotein. In addition, using HPLC-gel filtration, we have been able to demonstrate that NKCF consists of molecules in the molecular weight range between 20 and 30 kilodaltons. We have also been able to generate antibodies directed against NKCF. In rats, the antibodies inhibited both NK-CMC and inhibited NKCF cytotoxic reaction. Hybridoma derived from these still retain an inhibitory activity. At present, we are further purifying NKCF to homogeneity. We will also examine binding of NKCF by radiolabeled NKCF and the mechanism by which NKCF exerts its cytotoxic reaction. (LB)